Currently, communication in the frequency band used for radio communication is coming close to reaching a limit. In order to solve this problem, a communication technique has been studied in which Orbital Angular Momentum (OAM) is given to a radio signal, and the signal is formed into a helical beam for transmission and reception. The signal from which the helical beam is formed has a feature that the equiphase surface rotates in a helical manner. A change in a helical rotation pitch of the equiphase surface included in the helical beam enables a signal in an infinite orthogonal mode to be formed. Thus, when a helical beam is used for radio communication, a plurality of communications can be established at the same frequency, and communication can be performed at a high speed and with a large capacity.
Examples of documents relating to an antenna using signals for a helical beam provided with orbital angular momentum include Patent Literature 1 to 3. Patent Literature 1 discloses an antenna for OAM including N (N is an integer of two or greater) antenna elements arranged at equal intervals on a concentric circle. The antenna for OAM outputs signals radiated from the antenna elements with a phase difference and forms a helical beam to which an orbital angular momentum is given. Patent Literature 2 discloses an antenna device including a wave source that outputs a signal having linear polarization or circular polarization and an OAM filter that forms a signal output from the wave source into a helical beam to which an orbital angular momentum is given. Patent Literature 3 discloses a transmitting antenna including a plurality of first wave sources that transmit a plurality of helical beams having orbital angular momentum in a plurality of modes and a parabolic second wave source that reflects the plurality of helical beams.